1. Field of the Invention
The present invention generally relates to steam generators for commercial nuclear power plants and, more particularly, is concerned with apparatus and method for preventing vibration of the columns of steam generator tubes and originally-installed antivibration bars during operation of the steam generator.
2. Description of the Prior Art
Heat produced by fission in a nuclear reactor core of a nuclear power plant is transferred to a primary reactor coolant flowing through the reactor core. The primary reactor coolant then flows through steam generators of the nuclear power plant where it transfers the heat to a secondary feedwater which is transformed thereby into steam. The steam is used to generate electricity by driving a conventional steam turbine-electrical generator apparatus.
Each steam generator has a large bundle of U-shaped tubes. The high temperature primary reactor coolant from the reactor core enters the steam generator through a coolant inlet nozzle and exits therefrom through a coolant outlet nozzle, both of which are located in the lower portion of the steam generator, after flowing through the interior of the tubes. The secondary feedwater enters the steam generator through a feedwater inlet nozzle and exits therefrom through a feedwater exit nozzle, both of which are located in the upper portion of the steam generator, after flowing along the exterior of the tubes. Thus, the interior coolant and exterior feedwater flows are in heat exchange relationship with one another but separated by the walls of the tubes.
The U-shaped tubes in the bundle thereof are arranged in successive columns and rows, and supported at their lower open ends by sealed attachment to a tube sheet which is disposed transverse to the longitudinal axis of the steam generator. A series of tube supports arranged in axially spaced relationship to each other support the straight portions of the tubes in the columns and rows thereof. An upper tube support assembly is utilized to support the upper U-shaped portions of the tubes in the spaced columns and rows.
More particularly, the upper tube support assembly includes a plurality of retainer rings arranged around the outside periphery of the tube bundle in axial spaced relationship to each other. The retainer rings, like the tube supports, are arranged substantially transverse to the axis of the steam generator. Each retainer ring is generally of an oval shape which coincides with the outside periphery of the tube bundle at the particular location of the ring. Thus, the size of the oval of the retainer ring decreases with the distance toward the upper end of the tube bundle. The uppermost retainer ring, therefore, is relatively small inasmuch as it is located at the uppermost portion of the tube bundle where the shape of the tube bundle is rapidly converging.
Each of the retainer rings is connected to a plurality of antivibration bars which are typically originally installed between each column of the U-shaped portions of the tubes. In some steam generators, the antivibration bars take the form of a bar bent into a V-shaped configuration such that two legs are formed with an angle therebetween. The angles of the bars range from obtuse in the lower one of the bars to acute in the higher one of the bars. The V-shaped bars are inserted between the successive columns of the steam generator flow tubes with the free ends of the bars being rigidly attached to the opposite sides of the appropriate retainer ring.
The antivibration bars are intended to prevent excessive vibration of the individual steam generator tubes. It is well known that the vibrations in question are caused by the interaction of the flexibility of the flow tubes with flow of water and steam past the tubes, and that the U-shaped portions of the tubes are most severely affected by the vibrations. However, the mechanical aspects of the curved or bent portions of the tubes are the major obstacles in the way of a solution to the problem. The U-shaped tubes have dimensional tolerances associated with their outer diameters. There are also variations caused by ovalization of the tubes as a result of the bending. Furthermore, the spatial relationship between adjacent tubes is a variable, albeit within set design limits. Thus, there is a dimensional tolerance associated with the nominal spacing between steam generator tubes. There is also a dimensional tolerance associated with the outer dimensions of the prior art vibration bars. Also, the bars must be slightly smaller than the space between adjacent columns of tubes in order to permit insertion of the bars therebetween.
The combination of these tolerances and dimensional variances means that the presence of gaps persists between the originally-installed antivibration bars and the tubes of the steam generator. Any gaps are, of course, undesirable because they allow flow-induced vibration of the tubes and relative motion between the tubes and the originally-installed antivibration bars. The relative motion can cause the tubes and bars to interact over time resulting in damaging wear and subsequent failure to the tubes of the steam generator.
Up to the present, the approach to elimination of the gaps between the originally-installed antivibration bars and the steam generator tubes has been complete replacement of the bars with one or combinations of several different types of replacement antivibration bars designed for that purpose: that is, removal of the originally installed bars, and installation of a complete new set of bars. Appleman U.S. Pat. No. (4,640,342), Lagally U.S. Pat. No. (4,653,576) and Lagally et al U.S. Pat. No. (4,720,840) describe the various replacement antivibration bars and their installation.
While complete replacement of the originally installed antivibration bars is a step in the right direction, it is tedious and time consuming and thus costly to carry out. Its cost has been rejected by at least one utility as not justified by the remaining life of the steam generators. Consequently, a need still exists for a less costly, but effective, technique for antivibration bar gap elimination which would minimize the cost for utilities.